Error on USE_CLANG_PLATFORM_BUILD=

A blank USE_CLANG_PLATFORM_BUILD= on the command line for make causes
confusion between make and kati.  make ignores writes to variables set
on the command line, so envsetup.mk cannot set it to true,
USE_CLANG_PLATFORM_BUILD is still empty in soong.mk, and soong.variables
gets DeviceUsesClang: false.  kati gets USE_CLANG_PLATFORM_BUILD= in its
environment instead of on its command line, so envsetup.mk when parsed
by kati sets USE_CLANG_PLATFORM_BUILD=true.

Error out early if USE_CLANG_PLATFORM_BUILD is not true or false after
attempting to set it to true if it is empty.

Change-Id: I0e2731315711db9a6ac6ea2bf49c88bccacd35f7
diff --git a/core/envsetup.mk b/core/envsetup.mk
index c3cd3c5..1a79ce2 100644
--- a/core/envsetup.mk
+++ b/core/envsetup.mk
@@ -475,3 +475,9 @@
 ifeq ($(USE_CLANG_PLATFORM_BUILD),)
 USE_CLANG_PLATFORM_BUILD := true
 endif
+
+ifneq ($(USE_CLANG_PLATFORM_BUILD),true)
+ifneq ($(USE_CLANG_PLATFORM_BUILD),false)
+$(error USE_CLANG_PLATFORM_BUILD must be true or false)
+endif
+endif